For uplink of LTE-Advanced, which is an advanced form of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), support for transmission by a plurality of antennas of a terminal (SU-MIMO (Single. User-Multiple Input Multiple Output)) has been studied. SU-MIMO is a technique in which one terminal transmits data signals from a plurality of antenna ports at the same time and at the same frequency, and spatial multiplexing of the data signals is attained using virtual communication paths (streams) in a space.
To effectively improve performance by SU-MIMO, a base station needs to know the conditions of the propagation paths from the antennas of the terminal by using a sounding reference signal (hereinafter referred to as “SRS”). Therefore, the terminal needs to transmit the SRS from each antenna. The sounding means estimation of the quality of a propagation path between the terminal and the base station.
The SRS of LTE is time-multiplexed with data at the end symbol of a subframe and transmitted in order to estimate the channel quality of an uplink data channel.
In LTE, only one transmission antenna is used and only one SRS is transmitted to the terminal. When the terminal includes N antennas in LTE-Advanced, however, the overhead for the SRS transmission increases by N-fold, which may cause lack of the SRS area.
Consequently, to improve the number of SRSs that can be transmitted in a cell per unit time, i.e. the SRS capacity, a method of using code resources for a data demodulation reference signal (hereinafter referred to as “DMRS”) is known (see Non Patent Literature 1).
Non Patent Literature 1 discloses that the base station directs the terminal to use uplink scheduling information (also referred to as UL Grant) to transmit the SRS. FIG. 1 shows information notified in the UL grant to indicate transmission of a data signal of LTE. The UL grant includes a predetermined number of bits indicating frequency resource information of signal (RB assignment), an amount of cyclic shift for DMRS (hereinafter referred to as “CS amount”), and the like.
When the terminal is directed through the UL grant from the base station to transmit the SRS, the terminal transmits the SRS in an SC-FDMA (Single Carrier-Frequency Division Multiple. Access) symbol where the DMRS in a PUSCH (Physical Uplink Shared Manned) should be placed. As shown in FIG. 2, the DMRS is transmitted using a central symbol of each slot of one subframe (two-slot configuration), which is a scheduling unit of the data signal. The DMRS and SRS are signals using cyclic shift sequence Obtained by cyclically shifting data of the same sequence (ZC sequence). Therefore, the SRS can be code-multiplexed with the DMRS in the same band by using a CS amount different from that of the DMRS.
Thus, although the SRS is to be transmitted only using the end symbol of one subframe according to the specification of LTE, the SRS can also be transmitted using the central symbol of the slot (DMRS symbol position). Therefore, the SRS capacity can be improved.
In the following explanation, the SRS to be transmitted by the direction in the UL grant from the base station will be referred to as a scheduled SRS.
If a UL grant for transmitting the scheduled SRS is newly added, the terminal needs to detect two types of UL grants including the UL grant for data transmission, which makes the transmission/reception processing of the terminal and the base station complicated. Therefore, it is likely that the UL grant for scheduled SRS transmission also serves as the UL grant for data (DMRS) transmission shown in FIG. 1.
In this case, a parameter “MCS+RV” indicating the transmission size of data can be used for distinction between the data transmission and the scheduled SRS transmission. More specifically, transmission of the scheduled SRS can be indicated by setting the transmission size of data to 0.
The number of CS notification bits for SRS is 3 bits, just like the number of CS notification bits for data (Cyclic shift for DMRS in FIG. 1).